The present invention arises out of a need for improved materials to patch defects in wood, e.g., to fill knotholes, voids, splits and other defective areas in plywood veneers. Polyurethane compositions have been the most commonly used patching material, but they have important limitations. For one, the polyisocyanate in such compositions reacts with the water present in wood products, causing foaming that weakens the bond between a patch and the adjacent wood; reaction with water can be minimized by the use of certain metal-based catalysts, but the potential toxicity of such catalysts has limited their use. Secondly, conventional polyurethane wood-patching compositions degrade under the high temperatures used in laminating phenolic-saturated papers to plywood to provide decorative or weather- and chemical-resistant surfaces; temperatures of 275.degree. to 300.degree. F (135.degree.-150.degree. C) are applied for 5-10 minutes in such "overlay" procedures, and such conditions degrade existing polyurethane materials and leave a weak bond between the overlay and patched area.
Rapid-curing materials based on polyesters and epoxies have also been used to patch or fill wood, but those materials exhibit other disadvantages. A principal disadvantage is that their curing reaction is highly exothermic. The result is thermal degradation, discoloration, bubbling, and undesired shrinkage and cracking of the filled area, with a consequent loss of physical properties.
The opportunities for patching wood products would be greatly expanded by the availability of new materials that avoid the above deficiencies, as well as exhibit other properties needed in wood patching. Such properties include applicability by mass production techniques and formation of dimensionally stable weather-resistant patches that can be cut, shaped, sanded, adhered, nailed, and finished as wood products. Insofar as known, materials that both avoid the above deficiencies and exhibit the desired properties have been unavailable until the present invention.